1. Field of the Invention
This invention pertains to machine tools, and more particularly to apparatus for machining drill points.
2. Description of the Prior Art
Twist drills having different types of cutting points are well known. Drill points commonly used in industrial applications include conventional, helical, Racon, split, and Bickford points. The different points are designed to achieve maximum cutting rates in a variety of workpiece materials and configurations.
Before starting the drill pointing process, the drill is positioned to a known orientation with respect to a grinding wheel. Initially positioning the drill is commonly referred to as timing the drill. In some prior machines, the drill is gripped within a holder, and the holder and drill are positioned as a unit to time the drill. If the drill is gripped within a holder, means is usually provided with the machine for selectively releasing the drill from the holder. In other machines, the drill is gripped directly, and timing involves positioning only the drill relative to the grinding wheel.
Machine grinding of the various twist drill points requires complex three-dimensional relative motion between the drill and the grinding wheel. Normally, the grinding wheel rotates about a generally stationary axis, and the drill is cyclically fed against the wheel while the drill undergoes at least three components of motion during each cycle. The first component of motion is a constant velocity rotation of the drill about its longitudinal axis The second motion is a linear reciprocation of the drill in directions perpendicular to the drill longitudinal axis. That motion is commonly called the lift motion. The third motion is movements of the drill nominally along its longitudinal axis. That motion is known as the feed motion.
Lift and feed motions occur in timed relation to the rotation of the drill. Lift and feed motions also occur in cycles that correspond to the number of cutting edges on the drill point, with a cycle being repeated for each cutting edge during one revolution of the drill. Although the drill rotates about its longitudinal axis at a constant angular velocity, neither the lift nor the feed motions need be of constant velocity.
For some types of drill points, a fourth motion of the drill is required. The fourth motion is a rotation of the drill about an axis that is perpendicular to but that does not intersect the drill longitudinal axis. That motion is frequently referred to as a rock motion. Like the lift and feed motions, the rock motion is cyclically repeated for each cutting edge during a drill revolution about its longitudinal axis.
In some prior drill grinding machines, the lift, feed, and rock motions are imparted to the drill being ground by a series of cams, cam followers, links, and levers. The cams are rotated at a constant speed. The links and levers convert cam rotational motion into the appropriate lift, feed, and rock motions at the drill point. Different cams are used for different types of drill points and for different drill diameters. In addition, many of the various components, such as the links and levers, are adjustable to suit different drill points and diameters. Examples of prior drill grinding machines constructed generally according to the foregoing description may be seen in U.S. Pat. No. 4,365,444 and in the references cited therein. Additional examples of prior drill grinding machines are described in U.S. Pat. Nos. 3,067,548 and 3,656,264. Other commercial equipment for grinding drill points is marketed under the trademarks Winslow, Winslowmatic, and Bickford Point.
The drill grinding machine of U.S. Pat. No. 4,365,444, as well as other commercially available equipment, has enjoyed widespread acceptance. However, the cams, links, and other mechanical drive trains of the prior machines are undesirably expensive to manufacture and assemble. In addition, the prior drive mechanisms must frequently be adjusted to suit the type of point to be ground or the drill size, which results in downtime and decreased productivity.
Consequently, there is room for improvement in the design and operation of drill pointing machines.